Chapter 4

Nuclear Power, Solar Power, and Things Beyond

Nuclear Fission

Uranium exists in nature, usually as a pitchblende ore. Pitchblende is mildly radioactive from its load of uranium, which means that it spontaneously emits alpha particles. The level of radioactivity is very low, however, so there is no threat from mining and transporting the ore, and in any case, as we learned earlier, alpha particles pose no threat outside the body anyway. The uranium normally extracted from pitchblende typically takes two different forms, called isotopes: Uranium-235 and Uranium-238. The technical difference between them is that Uranium-238 has three extra neutrons in its nucleus than does Uranium-235. About 99.3 percent of uranium in the Earth's crust is Uranium-238; only about 0.7 percent is Uranium-235, along with a trace percentage of four other isotopes. This is important because the 235 isotope of uranium is the basis of most current nuclear power generation.

Uranium-235 is fissile, which means that it can fission thermally. This means that the nucleus can absorb a thermal neutron and then fission naturally into two pieces plus two or three neutrons plus energy. And a thermal neutron – well, that's a slow or low-energy neutron that has the same temperature as its surroundings; it is said to be in thermal equilibrium. The resulting pieces typically are various isotopes of barium, krypton, strontium, cesium, iodine, and xenon. Both the barium and krypton isotopes subsequently decay by emitting beta particles to form more stable isotopes of neodymium and yttrium. These beta decays, with some associated gamma-rays, make the fission products highly radioactive.

The two or three neutrons created by the fission are, themselves, captured to make the process happen all over again, and again, and again.... About 85 percent of the energy released in a nuclear reactor is carried in the motion of these byproducts. About 7 percent is generated by the radioactive decay of the byproducts.

Uranium-238 is not fissile; however, it can capture a thermal neutron, becoming Uranium-239. Almost immediately, it emits a beta particle, becoming Neptunium-239, which also immediately emits another beta particle. This results in Plutonium-239, which is relatively stable and is fissile. Plutonium-239 behaves in much the same way as Uranium-235, producing about one-third of the total energy in a nuclear reactor. Some of the Plutonium-239 captures a neutron to become less stable Plutonium-240, which captures another neutron to become Plutonium-241. This emits a beta particle to become Americium-241, which forms the heart of the modern household smoke detector.

So when you fuel a nuclear reactor with a mix of Uranium-235 and Uranium-238, you get a lot of energy in the form of heat, and some radioactivity as beta particles and gamma-rays. In current American reactors, the final products are a mixture of the elements listed earlier, plus isotopes of plutonium, neptunium, and americium. The latter three emit alpha particles for thousands of years.

(Part 3 of 9 follows)

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